We plan a fundamental study of multiple hydrogen bonding by acid-bases (species that are both acids and bases). We are particularly interested in multiple hydrogen bonding by lactams, imides, and amidines, including derivatives of 2-pyridone, 2-aminopyridine, and nucleic acid bases. We will study mono-hydrogen bonding by such species and/or their partially blocked derivatives, such as N-methyl-lactams and -imides. With models for nucleosides (9-alkyl-adenines and -guanines and 1-alkyl-uracils, -thymines, and -cytosines), where there is always more than one basic site that gets involved in multiple hydrogen bonding and often more than one acidic site, we will estimate hydrogen-bonding acidities and basicities of various sites by use of partly blocked compounds, correlations of hydrogen-bonding basicity (and acidity) with proton basicity (and acidity), and other means. From the results obtained with nucleoside models we hope to understand their specificity in hydrogen bonding to each other. We will also study substituent effects on multiple hydrogen bonding, including both dimerization and mixed complex formation.